Automated vehicle radar system with self-calibration

ABSTRACT

An automated vehicle radar system capable of self-calibration includes an antenna, a transceiver, and a controller. The antenna broadcasts a radar-signal and detects a reflected-signal reflected by an object. The transceiver determines a distance, an angle, and a range-rate of the object relative to the antenna based on the radar-signal and the reflected-signal. The controller determines a speed of a host-vehicle; determines when the object is stationary based on the speed, the angle, and the range-rate; stores in a memory a plurality of detections that correspond to multiple instances of the distance, the angle, and the range-rate as the host-vehicle travels by the object; selects an ideal-response of angle versus range-rate based on the speed; determines a calibration-matrix of the system based on a difference between the plurality of detections and the ideal-response; and adjusts an indicated-angle to a subsequent-object in accordance with the calibration-matrix.

TECHNICAL FIELD OF INVENTION

This disclosure generally relates to an automated vehicle radar system,and more particularly relates to a radar system that self-calibrates asa host-vehicle travels past a stationary object.

BACKGROUND OF INVENTION

Automotive radar systems are often initially calibrated in a testfacility such as an anechoic chamber, and then installed on ahost-vehicle without further re-calibration. As such, the calibrationmay not be optimal as the calibration does not compensate for theeffects of the fascia, frame, or other features of the host-vehicle thatmay influence the response of the radar system. Also, this initialcalibration does not compensate for changes of response due to aging orchanges in environmental conditions, which can and often do lead todeterioration in system performance. A typical initial calibration ofthe radar system prior to installation in a host-vehicle places a targetat a reference point in the chamber, and the radar antenna is rotated soa response is collected over a set of angles that correspond to theradar system's operating field-of-view. At each angle, a response in theform of complex voltages is collected for each element of the antenna,and these responses can be used to fully determine a default or initialcalibration of the system. That is, where the responses deviate from anexpected or ideal-response is noted and a correction factor orcalibration-matrix is established to correct or compensate thecomplex-voltages from each element of the antenna for each of the anglestested.

SUMMARY OF THE INVENTION

Described herein is a radar system that is programmed with a method foron-line or on-the-fly calibration of an automotive radar system, wherethe calibration does not rely on a controlled chamber environment orpredetermined test facility. The calibration can be performed while thehost-vehicle is traveling throughout the operating lifetime of thesystem. This provides for a more precise calibration of the system evenas parts of the system or the host-vehicle change (e.g. degrade) overtime and/or due to changes in operating environment. The system uses thepremise that a response from a single target can be found for multipleangles as the host-vehicle travels past the target. The method includesprocedures for picking out such targets and updating/tuning acalibration-matrix that provides for a full calibration of the system.The method make use of a Doppler scattering phenomenon where astationary object takes on different Doppler value due to the changingradial speed as the host-vehicle makes it way toward and eventually pastthe stationary object, where the radial speed is dependent on the speedof the host-vehicle and the angle to the object. The range of physicalangles of the stationary object depends on its relative position withrespect to the host-vehicle, which is tied to the initial range. Theremay be a one-to-one mapping between a certain Doppler value and physicalangle of the stationary object. For a forward facing radar, thestationary object of interest initially may be located at one degree(1°) offset from the bore-site of the system. As long as thehost-vehicle has a fixed trajectory (driving at a constant speed on arelative straight roadway) a stationary object such as a light pole, orspeed limit sign can be tracked and the data can be stored and laterused for calibration.

At certain intervals determined at the time of the design, the radargoes into an on-line calibration mode. This can be based on milestraveled, weather changes, a time-interval (periodic), or a time-deltafrom the initial time of sensor operation. In the calibration mode, thesystem may be switched into a sensing mode which gives it the highestDoppler resolution that is appropriate with the vehicle speed. Forexample, the system may be configured to gather separate detections ofthe object at the one degree angular intervals. For example, if thehost-vehicle traveling at 60 kph and the object is at an angle of onedegree (the most stressing), the system needs at least a 2.5 mm/sDoppler resolution to be able to determine that the object is at the 1degree location. However, when the object is at the 40 degrees, theDoppler resolution only needs to be 185 mm/s. detection range is limitedto 5-40 degrees instead of 1-40 degrees; the minimum Doppler resolutionrequirements can be relaxed.

After setting the parameters of the sensing mode based on the speed ofthe host-vehicle, the field-of-view of the system may be searched fordominant stationary object. The object is then tracked and complexvoltages recorded at, for example, one-degree angle intervals, andcontinue until the object exits the field-of-view of the system, forexample at forty-five degrees. It is noted that the collection of datadoes not need to be complete for every degree of angle in thefield-of-view, but the more complete the collection, the more exact thecalibration would become.

In accordance with one embodiment, a radar system suitable for use on anautomated vehicle and capable of self-calibration is provided. Thesystem includes an antenna, a transceiver, and a controller. The antennais mounted on a host-vehicle. The antenna is used to broadcast aradar-signal and detect a reflected-signal arising from a reflection ofthe radar-signal by an object. The transceiver is in communication withthe antenna. The transceiver determines a distance, an angle, and arange-rate of the object relative to the antenna based on theradar-signal and the reflected-signal. The controller is incommunication with the transceiver. The controller determines a speed ofthe host-vehicle; determines when the object is stationary based on thespeed, the angle, and the range-rate; stores in a memory a plurality ofdetections that correspond to multiple instances of the distance, theangle, and the range-rate as the host-vehicle travels by the object;selects an ideal-response of angle versus range-rate based on the speed;determines a calibration-matrix of the system based on a differencebetween the plurality of detections and the ideal-response when theobject is stationary; and adjusts an indicated-angle to asubsequent-object in accordance with the calibration-matrix.

Further features and advantages will appear more clearly on a reading ofthe following detailed description of the preferred embodiment, which isgiven by way of non-limiting example only and with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

The present invention will now be described, by way of example withreference to the accompanying drawings, in which:

FIG. 1 is a diagram of a radar system in accordance with one embodiment;

FIGS. 2A and 2B is an illustration of the relative locations of ahost-vehicle equipped with the system of FIG. 1 and a stationary objectdetected by the system of FIG. 1 a two different instance in time inaccordance with one embodiment; and

FIG. 3 is a graph of an ideal-response of the system of FIG. 1 inaccordance with one embodiment; and

FIG. 4 is a top-edge view of an antenna used by the system of FIG. 1 inaccordance with one embodiment.

DETAILED DESCRIPTION

FIG. 1 illustrates a non-limiting example of a radar system 10,hereafter referred to as the system 10. The system 10 is suitable foruse an automated vehicle, e.g. a host-vehicle 12. As used herein, theterm automated vehicle applies to instances when the host-vehicle 12 isbeing operated in an automated-mode 14, i.e. a fully autonomous mode,where a human-operator 18 of the host-vehicle 12 may do little more thandesignate a destination in order to operate the host-vehicle 12.However, full automation is not a requirement. It is contemplated thatthe teachings presented herein are useful when the host-vehicle 12 isoperated in a manual-mode 16 where the degree or level of automation maybe little more than providing an audible or visual warning to thehuman-operator 18 who is generally in control of the steering,accelerator, and brakes of the host-vehicle 12. For example, the system10 may merely assist the human-operator 18 as needed to change lanesand/or avoid interference with and/or a collision with, for example, anobject 20 such as an other-vehicle or a road sign.

The system 10 described herein is advantageously capable ofself-calibration to overcome the problems described above with regard tothe one-time initial calibration. As will be described in more detailbelow, the system 10 described herein is able to use detected instancesof stationary objects (e.g. road-signs or instances of reflectorsmounted on posts commonly used to indicate the edge of a roadway) toperiodically, regularly, or continuously update or refine thecalibration of the system 10.

The system 10 includes an antenna 22 mounted on a host-vehicle 12. Ingeneral, the antenna 22 has multiple instances of receive-elements 24(FIG. 4) so an azimuth angle to the object 20 can be determined, as willbe recognized by those in the art. The antenna 22 may be a singleunified unit built on a single substrate 26, or the antenna 22 may haveelements distributed at different spaced apart locations on thehost-vehicle 12. The antenna 22 may include one or more separateelements (not shown) used to broadcast a radar-signal 28, or one or moreof the receive-elements 24 may be multiplexed to broadcast theradar-signal 28 and detect a reflected-signal 30 arising from areflection of the radar-signal 28 by the object 20.

The system 10 includes a transceiver 32 in communication with theantenna 22. The communication may be by way of wires, fiber-optics,wave-guides, and the like, or any combination thereof, as will berecognized by those in the art. The transceiver 32 may be used todetermine a distance 34, an angle 36 (i.e. the aforementioned azimuthangle), and a range-rate 38 (see also FIGS. 2A and 2B) of the object 20relative to the host-vehicle 12, or more specifically the antenna 22,based on the radar-signal 28 and the reflected-signal 30.

The system 10 includes a controller 40 in communication with thetransceiver 32. While the transceiver 32 is shown as separate from thecontroller 40, this is only to simplify the explanation of the system10. It is contemplated that the function of the transceiver 32 could beintegrated into the controller 40. The controller 40 may include aprocessor (not specifically shown) such as a microprocessor or othercontrol circuitry such as analog and/or digital control circuitryincluding an application specific integrated circuit (ASIC) forprocessing data as should be evident to those in the art. The controller40 may include memory 42, including non-volatile memory, such aselectrically erasable programmable read-only memory (EEPROM) for storingone or more routines, thresholds, and captured data. The one or moreroutines may be executed by the processor to perform a method of stepsfor calibrating the system 10 based on signals sent and received by thecontroller 40 as described herein. That is, the controller 40 can bedescribed as being programmed to perform the method described below tocalibrate the system 10.

In one step the controller 40 determines a speed 44 of the host-vehicle12. The speed 44 may be determined in a number of ways including, butnot limited to, receiving a signal from a rotational-speed sensor (e.g.wheel speed sensor) where the signal may also be used to operate aspeedometer of the host-vehicle; receiving a speed-value from aglobal-positioning-system (GPS) of the host-vehicle 12; orcollecting/averaging range-rates associated with stationary-objectslocated near the bore-site 46 (FIGS. 2A and 2B) of the system 10.

In another step the controller 40 determines when the object 20 isstationary based on the speed 44, the angle 36, and the range-rate 38.If the range-rate 38 divided by the cosine of the angle 36 isapproximately equal to the speed 44, e.g. +/−2%, then the object 20 ispresumed to be stationary. If the object 20 is not stationary, it isrecognized that the object 20 could be tracked and used to calibrate thesystem 10. However, it is believed that the complexity and possiblelower confidence of using moving objects with unknown and inconsistentspeeds to calibrate the system 10 is not preferred as it is presumedthat there will be a sufficient number of stationary objects to make thecalibration of the system 10 effective. It is recognized that othercharacteristics (other than being stationary) of the reflected-signal 30reflected by the object 20 may be examined to determine if the object 20is suitable to use for calibrating the system 10, some of which will bedescribed later.

FIGS. 2A and 2B illustrate a non-limiting example of a scenario wherethe object 20 is located adjacent to a roadway or lane traveled by thehost-vehicle 12. FIG. 2A shows the relative locations of the object 20and the host-vehicle 12 at Time A. The object 20 is relatively distantso the angle 36 is relatively small. As such, the range-rate 38 will berelatively similar to the speed 44. In FIG. 2B at a later time (Time Bis after Time A) the object 20 is closer so the angle 36 is greater sothe range-rate 38 will have decreased relative to the speed 44. FIGS. 2Aand 2B illustrate how the range-rate 38 can vary as the host-vehicletravels at a constant speed past the object 20.

Referring again to the method or programming of the controller 40, inanother step the controller 40 stores in the memory 42 a plurality ofdetections 48 that may be in the form of complex-voltages 50 from thereceive-elements 24. In general, the complex-voltages 50 correspond toor are indicative of multiple instances of the distance 34, the angle36, and the range-rate 38 as the host-vehicle 12 travels by the object20. As a generic mathematical description for an antenna with Ninstances of the receive-elements 24 (N=8 for the example antenna shownin FIG. 4) and the number of detections corresponding to M angles wheredetections are collected, the data can be assembled it into an N×Mmatrix, which is shown below as Eq. 1.

$\begin{matrix}{X = {\begin{bmatrix}x_{11} & \ldots & x_{1M} \\\vdots & \ddots & \vdots \\x_{N\; 1} & \ldots & x_{NM}\end{bmatrix}.}} & {{Eq}.\mspace{11mu} 1}\end{matrix}$

While the description so far may be interpreted to suggest that theplurality of detections 48 used to determine the X matrix shown in FIG.1 are associated with a single instance of the object 20, this is not arequirement. That is, the plurality of detections 48 may includecomplex-voltages associated with instances of the distance 34, the angle36, and the range-rate 38 from a plurality of objects 20′. By way ofexample and not limitation, the X matrix may include detections frommultiple instances of reflectors mounted on posts commonly used toindicate the edge of a roadway.

FIG. 3 is a graph that illustrates a non-limiting example of anideal-response 52 of the system 10 for the range-rate 38 versus theangle 36. The ideal-response 52 corresponds to what the system 10 woulddetermine if the system 10 were perfectly calibrated, and thehost-vehicle 12 traveled on a straight road at a constant speed. Itshould be recognized that FIG. 3 corresponds to the cosine of the angle36 multiplied by the speed 44, which in this instance is sixtykilometers-per-hour (60 kph) or sixteen and two-thirds meters-per-second(16.67 m/s). FIG. 3 is relevant because in another step the controller40 selects an ideal-response 52 of angle versus range-rate based on thespeed 44. That is, if the speed 44 is some value other than 60 kph, theideal-response 52 is scaled accordingly. As will be explained in moredetail below, the calibration of the system 10 is based on a comparisonof the actual detections stored in Eq. 1 above to the ideal-response 52.

In another step the controller 40 determines a calibration-matrix 54 ofthe system 10 based on a difference between the plurality of detections48 and the ideal-response 52 when the object 20 is stationary, where thedifference is based on an angle difference between an indicated-angle 66at an indicated-range-rate 60 and an ideal-angle at an ideal-range-rateindicated by the ideal-response 52 for the condition that theideal-range-rate is equal to the indicated-range-rate 60. To this end, aset of steering vectors corresponding to idealized responses is definedby Eq. 2 and Eq. 3 that corresponds to the configuration of the antenna22.

$\begin{matrix}{{{a(\theta)} = {\frac{1}{\sqrt{N}}\begin{bmatrix}1 \\e^{{i{(1)}}\phi} \\\vdots \\e^{{i{({N - 1})}}\phi}\end{bmatrix}}}\;,\;{where}} & {{Eq}.\mspace{11mu} 2} \\{\phi = {\frac{2\pi}{\lambda}d\;{{\sin(\theta)}.}}} & {{Eq}.\mspace{11mu} 3}\end{matrix}$

Then for each of the M angles of arrival, an N×M matrix composed of Msteering columns for each of the angles is defined by Eq. 4.A=[a _(θ) ₁ . . . a _(θ) _(M) ]  Eq. 4.

For each of the angle of arrival, a complex multiplier that correspondsto an overall arbitrary gain and a phase is provided that helps furtheradjusts the response from those angle, which is shown below as Eq. 5 andis labeled the matrix Z.

$\begin{matrix}{Z = {\begin{bmatrix}\lambda_{1} & 0 & 0 \\0 & \ddots & 0 \\0 & 0 & \lambda_{M}\end{bmatrix}.}} & {{Eq}.\mspace{11mu} 5}\end{matrix}$

In another step the controller 40 adjusts an indicated-angle 66 to asubsequent-object in accordance with the calibration-matrix 54. In Eq. 6below, the calibration-matrix 54 (labeled C) acts upon a plurality ofdetections 48 (labeled X) that are associated with a subsequent-object56, i.e. an object different from the object 20 that was used todetermine the calibration-matrix 54, to be equal the ideal response 52multiplied by a set of complex values that helps adjust the idealresponse to that of the calibrated response. The resulting equation isCX=AZ  Eq. 6.

C and Z can both be determined by solving the following Eq. 7 which isan optimization problem solved by textbook minimization approach overall possible values of C and Z.min(CX−ZA)  Eq. 7.

Since it is an optimization problem, there may not be a unique solution.Note that the number of angles included in the measurement vector X doesnot need to be complete. As the column of X approaches the full set ofangles, the more accurate the calibration-matrix 54 becomes. That is,once the calibration-matrix 54 is determined, the effect is thatcontroller 40 adjusts an indicated-distance 58 and anindicated-range-rate 60 to the subsequent-object 56 in accordance withthe calibration-matrix 54 to provide an adjusted-distance 62 and anadjusted-range-rate 64 that corresponds to the actual distance andrange-rate to the subsequent-object 56.

As suggested above, it is recognized that other characteristics (otherthan being stationary) of the reflected-signal 30 reflected by theobject 20 may be examined to determine if the object 20 is suitable touse for calibrating the system 10. By way of example and not limitation,an instance of the plurality of detections 48 associated with the object20 may be used to determine the calibration-matrix 54 when aface-linearity 70 of the object 20 is less than a linearity-threshold72. As used herein, the face-linearity 70 is a measure of how muchvariation in distance is indicated if multiple returns or targets areassociated with the object 20. For example, if a road-sign such as aspeed-limit-sign had multiple returns because the road-sign was so closethat the angular resolution of the system could detect the same sign atmore than one value of the angle 36, the distance 34 of those multiplereturns could be compared. Since the road-sign is relatively flat, therewould be little variation in the distance 34 for each of the returns. Bycontrast, if the object 20 where a parked-vehicle on the shoulder of theroadway, a returns corresponding to a tail-light reflector and aside-view mirror would have substantially different distances. Such avariation in distance could complicate the process of determining thecalibration matrix, so the parked-vehicle would not be a preferredinstance of the object 20 used for calibration of the system 10. If thelinearity-threshold 72 were set to twenty centimeters (20 cm) forexample, then the face-linearity 70 of the parked-vehicle would likelybe greater than the linearity-threshold 72 so the parked-vehicle may notbe a suitable object to use for calibration. However, the face-linearity70 of the road-sign would likely be less than the linearity-threshold 72so the road-sign may be a suitable object to use for calibration.

By way of further example, an instance of the plurality of detections 48associated with the object 20 may be used to determine thecalibration-matrix 54 when the distance 34 is less than adistance-threshold 74, fifty meters (50 m) for example. If the object 20is too far away, e.g. greater than 50 m, the signal-to-noise ratio (SNR)of the return signal may be too low to provide a relatively consistentdetection. Also, at such distance the host-vehicle 12 may need to travela substantial distance before the angle 36 changes enough for the system10 to detect a different value for the angle 36. As such, it may bepreferable to only use objects that are closer than thedistance-threshold 74. It may also be preferable to only use objectsthat are detected with a SNR above some threshold, and/or when thereflected-signal 30 has a signal-strength greater than somestrength-threshold.

FIG. 4 illustrates a non-limiting example of the antenna 22. As notedabove, it is preferable if the antenna 22 includes a plurality ofreceive-elements 24 so that a direction (e.g. the angle 36) to theobject 20 can be determined. While the illustration only shows threeinstances of the reflected-signal 30, this is only to simplify theillustration, and it is expected that typically all of thereceive-elements 24 will receive or detect an instance of thereflected-signal 30. As such, each instance of the plurality ofdetections 48 includes complex-voltages 50 from each of the plurality ofreceive-elements 24. While the explanation above may be interpreted tosuggest that the plurality of detections 48 are in the form of thedistance, the angle 36, and the range-rate 38, it should be understoodthat this is conceptual to simplify the explanation of the system 10.The reality is that the plurality of detections 48 are typically in theform of the complex-voltages 50 received from each of thereceive-elements 24, so it follows that the calibration-matrix 54 isconfigured to transform the complex-voltages 50 to correct or compensatefor differences in the response of the system when compared to theideal-response 52.

Accordingly, a radar system (the system 10), a controller 40 for thesystem 10, and a method of operating the system 10 is provided. Thesystem 10 is an improvement over systems that are initially calibratedin a manufacturing environment, but do not have the ability to updateand/or fine tune the calibration of the system 10 ‘on-the-fly’ after thehost-vehicle 12 has left the factory.

While this invention has been described in terms of the preferredembodiments thereof, it is not intended to be so limited, but ratheronly to the extent set forth in the claims that follow.

We claim:
 1. A radar system comprising: an antenna mounted on ahost-vehicle, wherein the antenna is used to broadcast a radar-signaland detect a reflected-signal arising from a reflection of theradar-signal by an object; a transceiver in communication with theantenna, wherein the transceiver determines a distance, an angle, and arange-rate of the object relative to the antenna based on theradar-signal and the reflected-signal; and a controller in communicationwith the transceiver, wherein the controller is configured to determinea speed of the host-vehicle; determine that the object is stationarybased on the speed, the angle, and the range-rate; store in a memory aplurality of detections that correspond to multiple instances of thedistance, the angle, and the range-rate as the host-vehicle travels bythe object; select an ideal-response of angle versus range-rate based onthe speed; determine, in response to a determination that the object isstationary, a calibration-matrix of the system for a plurality of anglesof arrival to the antenna, said calibration-matrix based on a differencebetween the plurality of detections and the ideal-response; and adjustan indicated-angle to a subsequent-object in accordance with thecalibration-matrix.
 2. The system in accordance with claim 1, whereinthe controller is configured to adjust an indicated-distance and anindicated-range-rate to the subsequent-object in accordance with thecalibration-matrix.
 3. The system in accordance with claim 1, whereinthe difference is based on an angle difference between anindicated-angle at an indicated-range-rate and an ideal-angle at anideal-range-rate equal to the indicated-range-rate.
 4. The system inaccordance with claim 1, wherein an instance of the plurality ofdetections associated with the object is used to determine thecalibration-matrix when a face-linearity of the object is less than alinearity-threshold.
 5. The system in accordance with claim 1, whereinan instance of the plurality of detections associated with the object isused to determine the calibration-matrix when the distance is less thana distance-threshold.
 6. The system in accordance with claim 1, whereinthe plurality of detections includes instances of the distance, theangle, and the range-rate from a plurality of objects.
 7. The system inaccordance with claim 1, wherein the antenna includes a plurality ofreceive-elements, and each instance of the plurality of detectionsincludes complex-voltages from each of the plurality ofreceive-elements.
 8. The system in accordance with claim 7, wherein thecalibration-matrix is configured to transform the complex-voltages. 9.The system in accordance with claim 1, wherein the calibration-matrixcomprises distinct correction factors for each of the plurality ofangles of arrival.
 10. The system in accordance with claim 1, whereinthe plurality of angles of arrival in the calibration-matrix are definedby angle intervals between each of the plurality of angles of arrival.11. A method comprising: broadcasting a radar-signal; detecting areflection of the radar-signal by an object; determining a distance, anangle, and a range-rate of the object relative to an antenna used todetect the reflected-signal; determining a speed of a host-vehicle onwhich the antenna is mounted; determining that the object is stationarybased on the speed, the angle, and the range-rate; storing in a memory aplurality of detections that correspond to multiple instances of thedistance, the angle, and the range-rate as the host-vehicle travels bythe object; selecting an ideal-response of angle versus range-rate basedon the speed; determining, in response to determining that the object isstationary, a calibration-matrix for a plurality of angles of arrival tothe antenna, said calibration-matrix based on a difference between theplurality of detections and the ideal-response; and adjusting anindicated-angle to a subsequent-object in accordance with thecalibration-matrix.
 12. The method in accordance with claim 11, whereinthe method comprises adjusting an indicated-distance and anindicated-range-rate to the subsequent-object in accordance with thecalibration-matrix.
 13. The method in accordance with claim 11, whereinthe difference is based on an angle difference between anindicated-angle at an indicated-range-rate and an ideal-angle at anideal-range-rate equal to the indicated-range-rate.
 14. The method inaccordance with claim 11, wherein an instance of the plurality ofdetections associated with the object is used to determine thecalibration-matrix when a face-linearity of the object is less than alinearity-threshold.
 15. The method in accordance with claim 11, whereinan instance of the plurality of detections associated with the object isused to determine the calibration-matrix when the distance is less thana distance-threshold.
 16. The method in accordance with claim 11,wherein the plurality of detections includes instances of the distance,the angle, and the range-rate from a plurality of objects.
 17. Themethod in accordance with claim 11, wherein the antenna includes aplurality of receive-elements, and each instance of the plurality ofdetections includes complex-voltages from each of the plurality ofreceive-elements.
 18. The system in accordance with claim 17, whereinthe calibration-matrix is configured to transform the complex-voltages.19. The system in accordance with claim 11, wherein thecalibration-matrix comprises distinct correction factors for each of theplurality of angles of arrival.
 20. The system in accordance with claim11, wherein the plurality of angles of arrival in the calibration-matrixare defined by angle intervals between each of the plurality of anglesof arrival.